Internal protection circuits are provided in voltage regulators to prevent permanent damage that could occur under accidental overloads. Typically, protection against shortcircuits is provided by a current limit circuit, whereby the pass current flowing through a pass transistor is kept below a current limit threshold. For three-terminal voltage regulators, it is desirable for a current limit circuit to operate from the input-output voltage differential of the voltage regulator because the output terminal of the voltage regulator is used as a common reference. It is also desirable for a voltage regulator with a current limit circuit to have a low dropout voltage, typically in the neighborhood of 1 volt. Furthermore, it is desirable for the current limit threshold to have a negative temperature coefficient, so that the current limit threshold decreases as the temperature of the regulator increases.
Foldback circuits are also provided in voltage regulators to protect the pass transistor from second breakdown caused by thermal instabilities during high power operation. High power operation can result in the formation of hot spots within localized areas of the pass transistor, causing current conduction in the transistor to be non-uniform and concentrated at these hot spots, eventually leading to device burn-out. In order to avoid second breakdown, the device needs to be operated within its safe operating area under all operating conditions. A foldback circuit decreases the current limit threshold when the input-output voltage differential exceeds a given foldback threshold, thereby protecting the pass transistor from thermal runaway failure. As for the current limit circuit, it is desirable that a voltage regulator with a foldback circuit have a low dropout voltage, and that the foldback circuit operates from the voltage differential and has a foldback threshold with a negative temperature coefficient.